Tag: WA8TOD

One of the last challenges to be conquered with the µBITx is the unacceptably high intermodulation distortion produced in the IF stages on transmit.

The IMD seems to be sourced in the Termination Insensitive Amplifiers used as bi-directional IF amps at both 45 and 12 MHz.

Warren WA8TOD is experimenting with a prototype MMIC amplifier utilising the ERA-3SM+ (80 cents each on eBay) mounted on an RF prototyping board ($2.50 each from SV1AFN) that is the proper size and form factor to replace both transmit TIAs. The MMIC/board measure flat response from 300 Hz to well over 200 MHz, and will provide around 22dB of gain.

This test quantifies the IMD performance of the combo. The reference point on the SA is shifted +6 dB so that the readings reflect the power level that would be achieved with a single tone for convenience of interpretation.

Measurements were taken at Vcc of 3.3 volts and current draw was the recommended 35 ma.

The blue trace shows performance at 0 dBm out and is an acceptable -42 dBc (-36 dB minus 6 dB for a single tone carrier). Performance at – 10 dBm out, the level needed to drive the uBitx driver/PA chain, is an excellent -51 dBc (-45 dB minus 6 dB for a single tone carrier).

Yet to be determined is how to add the LM1117-3.3 regulator to hold the Vcc at the required level.

The following diagram from Henning DK5LV provides an insight into the final four stages of the µBITx PA chain. This stretches from Q90 through to the finals.

Warren WA8TOD has measured the gain and drive requirements of the transmit chain at operating frequency from Q90 through the finals. A sweep generator was inserted at C80 and the level adjusted first for a nominal 1 watt indicated on the wattmeter (Purple trace) and then five watts indicated (Blue).

First Test

This is a replacement for the 45 MHz transmit amp comprised of Q20 – Q22 on the µBITx main board. Warren removed C20 and C22 and used two short lengths of miniature coax to take the signal off-board to a 7 dB attenuator and then on to the MMIC amplifier, and then back onto the board.

The resulting MMIC amplifier gain was +16 dB to match that of the BI-DI amp on the stock µBITx.

Test 1

The yellow trace is before the amplifier board was inserted and the purple trace after. 3rd order IMD was reduced by nearly 10 dB over the stock µBITx by using the MMIC amp.

Second test

Warren restored the Q20 – Q22 amplifier and moved the MMIC amp to the Q40 – 42 amp with similar results. Here he found that he required the full 23 dB of gain provided by the MMIC to achieve the same level of main signal.

Test 2

Yellow and purple traces are as before with the new measurement indicated on the blue trace. Results are almost identical indicating replacing these two amps together would provide 19 – 20 dB of IMD improvement which would make the transmitter completely viable and, in fact, better than some commercial radios in terms of IMD.

In both cases the indicated power out from the two tone test was a little over 4 watts.

Self-oscillation test

Warren also tested this MMIC board for its susceptibility to oscillation. He connected a 60 dB attenuator between the input and output while feeding a signal into the input. He then gradually reduced the attenuation one dB at a time until oscillation was visible on a 1.5 GHz spectrum.

The amplifier broke into oscillation very reliably when the attenuation was stepped below -14 dB. Higher than that and it was completely stable.

In the course of these two tests the board was hanging unshielded about 3 inches from the PA heat sinks and the output was a little over 4 watts. In both cases he saw no indication of oscillation.

The filter is a 45MHz 15Khz type filter identical to the existing roofing filter in the µBITx and replacing R27 in the original circuit. Warren WA8TOD encapsulated the filter, transformer, and capacitor in shrink wrap and connected the ground of the filter to the nearby junction of R13 that is grounded.

Warren’s implementation of the filter uses a single transformer on the output of the crystal filter. A capacitor is also needed between the low impedance transformer output and the center tap of the mixer transformer. He soldered a 0.1 uF SMD, one side to the output pad of R27 and the other side served as a connection for the transformer low impedance output. No doubt a through-hole component could also be deployed.

The filter input is attached directly to the other R27 pad. Warren elected to not use a second transformer because the source is much closer to the filter impedance than it is to 50 ohms.

In Warren’s case the transformer does not dramatically reduce the spurs versus the bare filter. The primary effect, and one that is desirable, is that a much more reasonable level of audio drive is available. The radio is still susceptible to intermodulation distortion because the levels are increased closer to the stock configuration, giving only slightly reduced power output.

Other implementations of the anti-spur filter

A number of others have now implemented the anti-spurs 45 MHz filter and demonstrated that it reduces spurs. uBITx.net believes this modification should be included in the production of the µBITx immediately, in order to clean up the output on SSB.

Glenn VK3PE has made his own board to mount the components for the filter mod on, and Nick VK4PP has designed a board (which will be for sale or included as a freebie with his LPF board). Photos of their board designs follow:

100 mV audio drive, without the filter in place, gave very unacceptable IMD performance. It may well be in the case of the added filter that the stages preceding the filter have enough dynamic range to work at that level and it is simply compensating for the insertion loss of the filter itself. That can and will be confirmed with two tone IMD testing.

Listening to the recovered voice quality and decide if it is adequate.

Adding the filter has introduced low frequency rolloff for LSB and high frequency rolloff for USB. The change is less than 6 dB and may not be objectionable but that will be a subjective judgement.

28MHz results

24.9 MHz results

18 MHz

And a wider scan …

And finally, here is a wider span showing 15 through 10 meters harmonic performance. Warren’s unit has the onboard filters completely removed so this scan was made with an external 30 MHz LPF plus the new 45MHz R27 filter.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting.

If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting. If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.